1. Field of the Invention
The present invention relates to an optical disk reproducing device and an optical disk reproducing method capable of reproducing rewritable or write-once optical disks.
2. Description of the Related Art
Publicly-known rewritable optical disks are typified by DVD-RW, DVD+RW and CD-RW, and write-once optical disks are typified by DVD-R, DVD+R and CD-R. The rewritable optical disks and write-once optical disks will be referred to as a “recordable disk”, or simply as a “disk”, hereinafter. An optical recording/reproducing device for handling these types of recording disks generally comprises a spindle motor for rotating the disk, a disk chucking mechanism disposed at the end of a rotational shaft of the spindle motor, an optical head for irradiating laser light onto the disk surface in order to record or reproduce signals, an optical head moving mechanism for moving the optical head to desired track position or in the vicinity thereof on the disk, and so forth.
For an exemplary case where recording/reproduction is carried out using recordable DVD-RW or DVD-R, the optical disk recording/reproducing device first moves the optical head to an initial position, and activates focus servo and tracking servo at the initial position. The initial position herein is often set a little more outwardly (outer circumferential side of the disk) than a read-in area considering variations in mechanical accuracy of the device or in dimensional accuracy of the disk, or compatibility to reproduction of disks having different formats. The disk has a groove and a land previously formed thereon. The groove has been wobbled based on modulation signals (referred to as “wobble signals”, hereinafter) corresponding to control signals for the spindle motor and gate signals for detecting land pre-pits. The land has pre-pits (the aforementioned land pre-pits) for enabling precise positioning during recording on the disk, and for storing recording address and other information necessary for recording. The optical disk recording/reproducing device can detect an address on the optical disk at the initial position by demodulating an address signal from the wobble signals of the groove and from the land pre-pit signals of the land. The optical disk recording/reproducing device then generates information for moving the optical head (or laser spot position) to a target position where recording or reproduction is to be effected based on thus-detected address, and moves the optical head (or laser spot position) based on the generated information. The optical disk recording/reproducing device then locks the tracking servo and focus servo at the target position, and starts recording or reproduction of data. It is to be noted that the address signal of DVD+RW, CD-R and CD-RW, having no land pre-pit, is demodulated from the wobble signal.
Meanwhile, the optical disk recording/reproducing device for handling disks such as DVD-R and DVD-RW typically comprises a dedicated signal detection circuit for applying the tracking servo to the groove based on so-called radial push-pull system, and a demodulation circuit for demodulating the address signal from the land pre-pit signal.
On the other hand, disk reproducing devices for DVDs, such as DVD-video, DVD-ROM, DVD-R and DVD-RW (referred to as “optical disk reproducing device”, hereinafter), adopt the pit-tracking system by which tracking servo is applied to a track comprising a series of signal pits recorded on the disk surface based on so-called differential phase detection method. That is, the disk reproducing device of this type generally does not have any signal detection circuit for applying tracking servo to the groove or any address demodulation circuit. The optical disk reproducing system therefore can apply tracking servo to an area on the recordable disk where data is recorded (referred to as a “data-recorded area”, hereinafter), but cannot apply tracking servo to an area where data is not recorded (referred to as a “non-recorded area”, hereinafter). In other words, it is to be understood that the optical disk reproducing device cannot reproduce data recorded on the disk when it failed in detecting any data-recorded area at the initial position, that is, when it failed in applying tracking servo due to absence of the pit track at the initial position, even if data-recorded area actually resides on the optical disk.
On the other hand, Japanese Patent Application No. 10-172147 proposes an optical disk reproducing device capable of discriminating the data-recorded area from the non-recorded area (mirror surface) on the recordable disk, based on results of comparison between an amplitude hold level of an RF signal output from the optical head and a predetermined reference level. It is to be noted that the optical head of the optical disk reproducing device dedicated to reproduction of the optical disk is expressed as an optical pickup.
More specifically, the optical disk reproducing device described in Japanese Patent Application No. 10-172147 moves the optical pickup to a predetermined detection point (referred to as a “first detection point”, hereinafter), and compares an amplitude hold level of an RF signal output from the optical pickup at the first detection point with a predetermined reference level. If the amplitude hold level is lower than the reference level, that is, if an area on the optical disk corresponded to the first detection point is the non-recorded area, the optical disk reproducing device shifts the optical pickup by a predetermined distance (5 mm, for example) towards the center of the disk, and at that point (referred to as a “second detection point”, hereinafter), compares again the amplitude hold level of the RF signal from the optical pickup with the reference level. If the amplitude hold level obtained at the second detection point is found to exceed the reference level, the optical disk reproducing device again pushes the optical pickup back towards the outer circumferential side of the disk by half of the aforementioned predetermined distance (2.5 mm, for example), and, assuming that position as a new first detection point, compares again the amplitude hold level with the reference level. The optical disk reproducing device repeats such shifting of the detection points and level comparison until the amplitude hold level exceeds the reference level. When the amplitude hold level exceeds the reference level at any detection point, that is, when the data-recorded area is detected, the optical disk reproducing device immediately starts data reproduction at that detection point.
The optical disk reproducing device described in Japanese Patent Application No. 10-172147 can discriminate whether the optical pickup falls on the data-recorded area or not, through the aforementioned shifting of the detection points and level comparison, and is designed so that the optical pickup, even if fallen on the non-recorded area, can escape therefrom so as to detect the data-recorded area.
Meanwhile, recent growing demands are directed to an optical disk reproducing device having a more advanced accuracy in detecting a boundary position between the data-recorded area and non-recorded area. If precise detection of the boundary position is realized, it becomes possible for the optical disk reproducing device not only to specify a target position for starting reproduction more rapidly and more accurately, but also to reproduce recorded data even when the amount of recorded data is extremely small and thus the width of the data-recorded area (width in the radial direction of the disk) is extremely narrow.
Referring now to FIG. 1, if a center hole 101 of a disk 100 is decentered from the center of rotation 102 of the disk 100, or if the center position of chucking of the disk is decentered from the center of rotation 102 for example, nonconformity between the center of rotation 102 of the disk 100 and the center of a rotational shaft of a spindle motor consequently occurs. This status will be expressed as “the disk 100 is decentered”. It is also to be noted that the amount of nonconformity between the center of rotation 102 of the disk 100 and the center position of the center hole 101 or the amount of nonconformity between the center of rotation 102 and the center position of chucking is expressed as “the amount of decentering of the disk”.
When the disk 100 is decentered as described above, a locus of laser spot irradiated on the disk 100 under rotation periodically swings (wobbles) towards the inner and outer circumferential sides of the disk 100 by a distance corresponded to the amount of decentering as shown by a trace pattern TPa in FIG. 2. On the contrary, when the disk 100 is not decentered, the locus of laser spot does not wobble neither inwardly nor outwardly (amount of decentering=0) as shown by a trace pattern TPb in FIG. 2. It is to be noted that the individual disk positions A, B, C and D in FIG. 1 correspond with the disk positions A, B, C and D in FIG. 2, respectively.
When the center hole 101 is placed as being decentered towards the disk position D, the locus of laser spot on the rotating disk 100 will be such that the disk positions A and C show the amount of decentering of 0, the disk position B shows an outward dislocation by an amount of decentering, and the disk position D shows an inward dislocation by an amount of decentering. In the exemplary case of FIGS. 1 and 2, disk positions B and C express dislocation turning points where the locus of laser spot is directed outwardly or inwardly. The dislocation turning points also express points of change where the relative speed between the laser spot and the disk increases or decrease.
Therefore, in the case where the disk 100 is decentered as in the example of FIGS. 1 and 2, if any laser spot resides within an area having a width corresponded to the amount of decentering from the boundary position (referred to as a “boundary area”, hereinafter), the laser spot consequently travels through the data-recorded area and non-recorded area in an alternate manner in accordance with the rotation of the disk 100. In particular when the amount of decentering of the disk 100 becomes relatively large, the boundary area is also widened, and this makes the laser spot more likely to travel through the data-recorded area and non-recorded area in an alternate manner.
In this case, it is very difficult for the optical disk reproducing device to detect the boundary position, and in the worst case, tracking servo cannot be activated in the data-recorded area, and this may cause hung-up of the servo control. Therefore, if the boundary area (having a width corresponded to the amount of decentering) can be accurately isolated from the data-recorded area, the optical disk reproducing device can certainly apply the tracking servo in the data-recorded area after being isolated from the boundary area, and can also rapidly detect the boundary position. Accurate isolation of the boundary area from the data-recorded area is also advantageous in that the optical disk reproducing device can certainly apply the tracking servo even when the data-recorded area has a width only slightly larger than that of the boundary area (width corresponded to the amount of decentering), and, as a consequence, in that data recorded in the data-recorded area can successfully be reproduced.